Synthesis of Ag/N-TiO2/SBA-15 Photocatalysts and Photocatalytic Reduction of CO2 under Visible Light Irradiation

Silver-loaded together with nitrogen-doped highly dispersed TiO2/SBA-15 mesoporous catalysts were successfully synthesized through solvothermal treatment and calcination method by using titanium n-butoxide (TB), carboxylate-modified SBA-15 (COOH/SBA-15), urea and silver nitrate as raw materials. The detailed preparation procedure was as follows: 1.0 g COOH/SBA-15 powder was dispersed in solution containing 0.82 mL titanium n-butoxide (TB) and 20.0 mL acetic ether. The mixture was stirred for 30 mm at room temperature, then 0.031 g AgNO3 was added into the solution by further stirring for 30 mm and a certain amount of urea was added. After being stirred continuously for 3 h, the resultant solution was transferred into a teflon-lined stainless autoclave and solvothermally treated at 220 degrees C for 12 h. After naturally cooling to room temperature, the resultant product was washed three times with absolute ethanol and then dried under vacuum at 80 degrees C for 12 h. Finally the solids were calcined in air at 550 degrees C for 4 h at a heating rate of 1.5 degrees C/min. The prepared samples were characterized by X-ray diffraction (XRD), N-2 adsorption-desorption, X-ray photoelectron spectrum (XPS), transmission electron microscopy (TEM), UV-Visible diffuse reflectance spectroscopy (UV-vis DRS), photoluminescence spectra (PL), inductively coupled plasma atomic emission spectrum (ICP) and elemental analysis (EA). It was revealed that the highly dispersed samples consist of anatase crystalline phase were mesoporous structure. The anatase phase was retained without phase change after silver loading and nitrogen doping. Silver was deposited on the surface of catalysts in the form of metallic silver and served as an effective electron trapper which could prevent the fast recombination of the photo-generated electrons and holes, at the same time, visible light absorption of samples were enhanced by silver nanoparticle based on its surface plasmon resonance effect. Nitrogen was doped into TiO2 matrix in the form of both interstitial nitrogen and substitutional nitrogen, nitrogen dopants might be presented in the chemical environment of Ti-O-N and O-Ti-N. On one hand, nitrogen doping could extend the adsorption of catalysts to the visible light region (lambda>400 nm); on the other hand, appropriate amount of nitrogen doping could inhibit the recombination of the photo-generated charge carriers and increase the efficiency of the photocurrent carriers. The high photocatalytic performance could be attributed to the synergistic effect of nitrogen doping and the loaded silver nanoparticles over TiO2. The nitrogen doping concentration had an optimal value. When the loading amount of Ag was 2 wt% and the theoretical mole ratio of nitrogen to Ti was 3, the photocatalyst had the highest photocatalytic activity. The methanol yield was 45.7 mu mol.g(-1).h(-1).